Nanostructures and microstructures, possibly including nanoparticles, on glass substrates are used for a variety of applications in display, lighting, architecture and photovoltaic devices. In display devices the structures can be used for enhanced light extraction or distribution. In lighting devices, the structures can be used for enhanced light extraction, distribution, and decorative effects. In photovoltaic devices, the structures can be used for solar concentration and anti-reflection. Patterning or otherwise forming nanostructures and microstructures on large glass substrates, particularly with nanoparticles, can be difficult and not cost-effective, motivating the present invention.
The majority of the light generated in the emissive layer of OLEDs is trapped by total internal reflection and waveguiding effects in the OLED layers and glass substrate. Light entrapment can lead to up to ˜80% reduction in efficiency of OLED displays. The loss of efficiency, in turn, translates to lower luminance, increased power consumption and shorter display lifetime and battery life.
A number of approaches have been demonstrated to improve the light extraction efficiency. These are typically based on either diffractive or optical scattering mechanisms, and include two-dimensional or 3D photonic crystal (PC) structures, roughened interfaces or textured surfaces, reflecting surfaces and distributed Bragg reflectors, nanoporous films, or the use of resonant microcavity structures.
U.S. Pat. No. 8,692,446 describes a novel light out-coupling construction that, when incorporated in an organic light-emitting device (OLED), provides enhanced light extraction as well as improved angular and spectral uniformity. The out-coupling construction can be used as a substrate for OLED fabrication, and contains light extraction features consisting of coatings of nanoparticles (0.1˜0.8 μm in diameter) applied to a polymeric substrate pre-patterned with one-dimensional or two-dimensional periodic structures. This patent also demonstrates that the nanoparticle/grating structure can be planarized with a high-index coating such as silicon introduce to match the refractive index of the transparent conductor used in the OLED, typically Indium Tin Oxide.
These approaches can provide following advantages over pure one-dimensional gratings and two-dimensional grating structures as well as a coating of the nanoparticles alone. For example: The presence of the 1D or 2D periodic structures can improve the uniformity of the nanoparticle coatings; and compared to purely periodic extraction features, the combination of nanoparticles with the 1D or 2D periodic structures can improve the angular and spectral uniformity.
However, the polymer used for the 1D grating may not be dimensionally stable during the process of creating the OLED display stack, in which high temperatures are used to anneal the indium tin oxide transparent conductor. There is also a number of coating steps that are needed in order to manufacture this structure which will raise the cost of manufacturing the product.
In U.S. Patent Application Publication No. 2014/0021492, nanostructured lamination transfer films are described that enable the fabrication of nanostructured solid surfaces, using simple lamination and bake out steps. The invention detailed the use of a microreplicated polymer film, whose micro-cavities can be filled with an inorganic polysiloxane coating. After curing the polysiloxane to a green state, the stack can be transferred to a glass slide with the microreplicated polymer film facing up. The entire construction is then baked in an oven to remove the polymer film and completely condense the inorganic polysiloxane. The resulting inorganic coating imparts microstructure to the glass slide, in the form of the inverse “daughter” of the microstructure imparted in the polymer film.
We propose an improved method to create self-aligned silica nanoparticles within a one-dimensional silica grating to produce optically functional inorganic coatings. To create the structure, a roll-to-roll coated precursor film could be applied to a substrate via lamination and bake steps. During the bake step, the nearly monodisperse nanoparticles settle into the microreplicated structure formed by the siloxane coating.